In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and lithographic ink is applied, the hydrophilic regions retain the water and repel the lithographic ink the ink receptive regions accept the lithographic ink and repel the water. The lithographic ink is then transferred to the surface of suitable materials upon which the image is to be reproduced. In some instances, the lithographic ink can be first transferred to an intermediate blanket that in turn is used to transfer the lithographic ink to the surface of the materials upon which the image is to be reproduced.
Lithographic printing plate precursors useful to prepare lithographic (or offset) printing plates typically comprise one or more imageable layers applied over a hydrophilic surface of a substrate (or hydrophilic intermediate layers). The imageable layer(s) can comprise one or more radiation-sensitive components dispersed within a suitable binder. Following imaging, either the exposed regions or the non-exposed regions of the imageable layer(s) are removed by a suitable processing solution (developer), revealing the underlying hydrophilic surface. If the exposed regions are removed, the precursor is considered as positive-working. Conversely, if the non-exposed regions are removed, the precursor is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water or aqueous solutions (typically a fountain solution), and repel lithographic ink.
Various negative-working imageable elements have been designed for processing or development “on-press” using a fountain solution, lithographic printing ink, or both. For example, such elements are described in U.S. Patent Application Publication 2005-263021 (Mitsumoto et al.) and in U.S. Pat. No. 6,071,675 (Teng), U.S. Pat. No. 6,387,595 (Teng), U.S. Pat. No. 6,482,571 (Teng), U.S. Pat. No. 6,495,310 (Teng), U.S. Pat. No. 6,541,183 (Teng), U.S. Pat. No. 6,548,222 (Teng), U.S. Pat. No. 6,576,401 (Teng), U.S. Pat. No. 6,899,994 (Huang et al.), U.S. Pat. No. 6,902,866 (Teng), and U.S. Pat. No. 7,089,856 (Teng).
U.S. Pat. No. 7,429,445 (Munnelly et al.) describes on-press developable negative-working lithographic printing plate precursors that contain various infrared radiation absorbing dyes that have tetraaryl pentadiene chromophores, and nonionic phosphate acrylates to increase imaging sensitivity. By “on-press” developable, it is meant that the imaged precursor does not need a separate processing step for development before it is placed on a printing press. Thus, processing or development can occur during the first few printed impressions on the printing press.
U.S. Patent Application Publication 2009-0269699 (Munnelly et al.) describes the use of IR absorbable dyes to improve color contrast in on-press developable lithographic printing plate precursor.
In general, lithographic printing plates contain a colorant (dye or pigment) in the radiation-sensitive composition (imageable layer) that has the function of making the image visible after processing in order to be inspected and evaluated for plate positioning and bar code readability prior to pre-press calibration. Such colorants provide contrast between the image and the background. This image contrast (print out) is particularly needed for lithographic printing plate precursors designed for development on-press. However, certain lithographic printing plates cannot contain a colorant for different reasons. For example, the imaged lithographic printing plate precursors that are usually developed on-press have a colorless coating because if a colorant is present, it could contaminate the lithographic printing ink and the fountain solution used for development and printing, with the result of altering the printed color shades. However, sometimes it is necessary for such printing plates to be used the same way as those developed off-press. In such instances, the image needs to be seen and evaluated with sufficient printout.
U.S. Patent Application Publication 2010-0227669 (Simpson et al.) describes the use of a visible pigment and dye mixture for color contrast. U.S. Pat. No. 8,034,538 (Strehmel et al.) describes the use of water-soluble visible contrast dyes.
For imaged elements that are to be developed on-press, the image is not easily identified. Adding colorant to on-press developable imaging compositions compromises plate shelf life, on-press developability, or imaging sensitivity, and the colorant may color-contaminate printing press inks. Thus, there is a need for an adequate print-out that provides visibility to the image on the printing plate before on-press development. Simply increasing imaging energy beyond that required for image durability may improve the printout, however, with a resulting increase in dot gain. Furthermore, increasing the exposure energy reduces productivity for the customer, which means that the number of printing plates that can be produced per hour is decreased.
U.S. Patent Application Publications 2005/0170282 (Inno et al.), 2005/0233251 (Kakino et al.), 2003/0068575 (Yanaka), 2006/0046189 (Kunita et al.), and 2007/0072119 (Iwai et al.), and U.S Patent Application Publications 2008/0311524 (Callant et al.), 2010/0040976 (Callant et al.), and 2005/0271976 (Kakino et al.) describe lithographic printing plate precursors that contain a discoloring agent or system capable of generating a color change upon exposure for providing print-out.
U.S. Patent Application Publication 2009/0047599 (Horne et al.) describes the use of specific spirolactone or spirolactam leuco dye color formers in the imageable layer of negative-working imageable elements.
U.S. Pat. No. 8,247,163 (Memetea et al.) describes the application of a coloring fluid to the imaged precursor such that the optical density in the solid exposed regions is OD2 that is greater than OD1. This coloring fluid comprises a water-insoluble colorant and a solvent that is capable of swelling the solid exposed regions of the imageable layer.
U.S. Patent Application Publication 2010-0227669 (Simpson et al.) describes the use of a visible pigment and dye mixture for color contrast. U.S. Pat. No. 8,034,538 (Strehmel et al.) describes the use of water-soluble visible contrast dyes.
The desired feature of good run length is influenced by the degree of polymerization or crosslinking of the ethylenically unsaturated components in the imageable layer. Additives that enhance printout usually do not participate in the crosslinking reactions and therefore do not contribute to the run length and could even be responsible to reducing run length. Therefore it is a goal to reduce the number of components that do not participate in the crosslinking. For thermal photopolymer imageable layers that require an infrared radiation absorber for image formation, the selection of components is determined to a large extent on obtaining high lithographic printing plate precursor sensitivity and shelf life. Therefore an infrared radiation absorber that not only provides high sensitivity and shelf life but also is effective to provide a strong color change is highly desirable. It is a further desire to obtain these features with a single absorber without a need for combination of infrared radiation absorbers.